Last Updated on May 30, 2026 by John Berry
The Moon’s orbit is locked to that of Earth, so the same part (or face) of the Moon always presents to observers on Earth. But there’s an additional effect. The Moon nods and wobbles. Astronomers are hugely interested in this movement, termed libration, because it allows them to see a bit more of the surface around the edges. That’s of no interest to the radio amateur. But libration causes varying reflection and scattering of EME signals that results in varying degrees of libration fading.
The transmitted wavefront strikes millions of discrete boulders, craters, ridges, and plains. How the waves are returned by this geography depends on the terrain roughness and that’s a factor of wavelength. The lunar surface appears substantially smooth at VHF with larger wavelengths resulting in near-specular reflection. Roughness, and hence liberation fading, rises with frequency as reflection turns to scattering.
The received signal back on Earth is the vector sum of these multiple distinct reflections and scatters. As the Moon nods and wobbles the pattern of reflections and scatters changes. As a result, the relative amplitudes and phases of these returns shift continuously . This gives a changing pattern of constructive and destructive interference at the receiver.
Libration fading
As is implied by the term fading, the the signal returned to Earth is not at a steady level. By way of analogy, a car moving in a city, using a receiver tuned to a broadcast FM radio station (or any other for that matter), receives multiple signal reflections from its surroundings. The aggregate received signal follows a Rayleigh frequency distribution with deep fades at a period of half a wavelength. The signal level with position is shown below. Position is that of the car doing the receiving.
This Rayleigh fading signal is described by its median value (value exceeded for 50% of time) and its variance. There is also typically a significant slow fading component and the aggregate signal follows a combined Rayleigh and log-normal (or Ricean) distribution.
Unlike the car, EME stations are fixed on the surface of the Earth. But the reflecting environment – the Moon – is moving relative to those stations and hence the same sort of multi-path fading occurs. The Moon’s libration movement is the equivalent of someone walking through a city at about 5km/hr while sensing the cusps and nulls. The Earthly receiver senses a fast fading rate of half a second at 144MHz. The fade rate rises with frequency. There is also a significant diurnal slow fading component.
Changes in libration
There are times when libration is a minimum, and others when it’s maximum. The corresponding libration fading reduces to give a steadier signal received at the minimums, but at the maximums the deep fades are annoying to EME enthusiasts. The image below of the Moon’s libration shows two periods when the libration is worst, at apogee and perigee, around the points of inflexion. And it shows two periods when it’s least, when the Moon is in its orbit between 370,000km and 400,00km distant.
Nodding is change around the east-west Moon axis. Wobbling is change around the north-south axis. Both nodding and wobbling occur with slightly different periods of around 27 days. The image below repeats the argument that libration is minimum halfway between perigee and apogee. Since the movement is then least, that’s the best time for radio hams to attempt EME communications. That’s from the perspective of libration fading – there are other effects that don’t necessarily coincide with libration.
Libration and coding
Sensed in the amplitude domain, the level fades up and down. In the frequency domain, the spread in time of the various returns gives rise to a small Doppler spread. Since radio amateurs use relatively narrow bandwidth transmissions, the effect of Doppler spread is minimal.
For transmission systems to cope well with fading, they must consider libration. Fast fading has cusps rising above the median, and then deep fades half a wavelength apart. This gives fade rates of 0.5 fades per second at 144MHz rising to and 5 fades per second at 1296MHz. Amateurs have measured the cusps as giving up to 10 dB of enhancement and up to 23dB degradation over EME paths. It’s a pretty hostile environment in which to communicate.
Classically, communications engineers align the data coding – the way the data is impressed on the carrier signal – with the nature of the signal received. The approach used by radio amateur designers is to acknowledge the fades, and resend the information over and over within the transmission period. Riding on multiple cusps, eventually the information will be received. I discuss the nature of coding systems like Q65 in a presentation on data modes.
Ultimately, system designers quote receiver thresholds at a frequency and for a given coding system taking account of the nature of the received signal.